We need to quiesce interrupts in the poll_get_char routine, otherwise,if used with KGDB NMI debugger, we'll keep reentering the NMI.

Quiescing interrupts is pretty straightforward, except for TXIMinterrupt. The interrupt has "ready to transmit" meaning, so it'salmost always raised, and the only way to silence it is to mask it. Butthat's OK, ops->start_tx will unmask it.